Jason M. Wilham

Simplified ultrasensitive prion detection by recombinant PrP conversion with shaking

To the editor: A key problem in managing prion diseases is the lack of a rapid, practical assay for prions (infectivity) at low-level infectious, or sub-infectious, amounts. Prion diseases involve the accumulation of a pathological, typically protease-resistant form of prion protein, termed PrPSc, which appears to propagate itself in infected hosts by inducing the conversion of its normal hostencoded precursor, PrP-sen, into additional PrPSc (refs. 1–4). In crude brain homogenates, PrPSc and infectivity can be amplified from endogenous PrP-sen during multiple rounds of intermittent sonication and serial dilution into fresh normal brain homogenate2,4. This ultrasensitive assay, termed PMCA, allows detection of ~1 ag of PrPSc in ~3 weeks5. To improve the speed and practicality of prion detection assays, we recently developed a cell-free conversion reaction that supports sustained PrPSc-seeded conversion of recombinant PrP-sen (rPrP-sen) to specific protease-resistant (rPrP-res) forms. This method (which we previously reported in Nature Methods), called rPrP-PMCA, uses periodic sonication and serial reaction rounds of the PMCA method, but is faster6. To circumvent problems associated with sonication in the PMCA and rPrP-PMCA methods (see Supplementary Results online), we have now developed a new prion assay, abbreviated QUIC for quaking-induced conversion, which uses rPrP-sen as a substrate and automated tube shaking rather than sonication. This assay can detect about one lethal prion dose within a day, and is faster and simpler than previous described PMCA6 and rPrP-PMCA5 assays. Initial testing of QUIC reaction conditions revealed that periodic shaking enhanced PrPSc-seeded conversion of hamster rPrP-sen (residues 23–231) into PK-resistant conversion products (rPrP-res(Sc), where (Sc) refers to seeding by PrPSc; Supplementary Fig. 1 and Supplementary Methods online). Consistent with our previous observations with rPrP-PMCA reactions6, the rPrP-res(Sc) reaction products had 17-, 13-, 12and 11-kDa fragments, which represented different C-terminal PrP fragments (Supplementary Fig. 2 online). These results showed that periodic shaking could substitute for sonication in promoting rPrP-res(Sc) formation. Additional experiments revealed that rPrP-res(Sc) generation was also sensitive to rPrP-sen concentration, reaction volume (Supplementary Fig. 1), reaction time (Supplementary Fig. 2), number of serial reactions (Supplementary Fig. 3 online), temperature (Supplementary Fig. 4 online) and shaking cycle (Supplementary Results). In QUIC reactions performed at 45 °C, we observed rPrP-res(Sc) formation in single 46-h QUIC reactions seeded with ≥100 ag of PrPSc (Fig. 1a). In contrast, 21 negative control reactions seeded with comparable dilutions of normal brain homogenate or buffer alone produced no rPrP-res (Fig. 1b). We obtained results similar to those shown in Figure 1a,b in an independent repeat experiment done in triplicate (data not shown). When we diluted products of PrPSc-seeded reactions 1,000-fold into fresh rPrP-sen to seed the subsequent reaction rounds, we observed strong propagation of rPrP-res(Sc) through at least 4 serial reactions (Supplementary Fig. 5 online). Elevation of QUIC reaction temperatures accelerated rPrP-res(Sc) formation. At 55 °C, we detected rPrP-res(Sc) in single 8-h reactions seeded with as little as 10 fg PrPSc (~2 lethal intracerebral doses; Supplementary Fig. 4). We detected 1 fg in 18-h reactions (Supplementary Fig. 6 online). At 65 °C, we detected 100 fg PrPSc seed with a 4-h reaction (Supplementary Fig. 4). However, at 65 °C, there was also 25 20

Real time quaking-induced conversion analysis of cerebrospinal fluid in sporadic Creutzfeldt-Jakob disease

Current cerebrospinal fluid (CSF) tests for sporadic Creutzfeldt–Jakob disease (sCJD) are based on the detection of surrogate markers of neuronal damage such as CSF 14‐3‐3, which are not specific for sCJD. A number of prion protein conversion assays have been developed, including real time quaking‐induced conversion (RT‐QuIC). The objective of this study is to investigate whether CSF RT‐QuIC analysis could be used as a diagnostic test in sCJD.

Prion Disease Blood Test Using Immunoprecipitation and Improved Quaking-Induced Conversion

ABSTRACT A key challenge in managing transmissible spongiform encephalopathies (TSEs) or prion diseases in medicine, agriculture, and wildlife biology is the development of practical tests for prions that are at or below infectious levels. Of particular interest are tests capable of detecting prions in blood components such as plasma, but blood typically has extremely low prion concentrations and contains inhibitors of the most sensitive prion tests. One of the latter tests is quaking-induced conversion (QuIC), which can be as sensitive as in vivo bioassays, but much more rapid, higher throughput, and less expensive. Now we have integrated antibody 15B3-based immunoprecipitation with QuIC reactions to increase sensitivity and isolate prions from inhibitors such as those in plasma samples. Coupling of immunoprecipitation and an improved real-time QuIC reaction dramatically enhanced detection of variant Creutzfeldt-Jakob disease (vCJD) brain tissue diluted into human plasma. Dilutions of 1014-fold, containing ~2 attogram (ag) per ml of proteinase K-resistant prion protein, were readily detected, indicating ~10,000-fold greater sensitivity for vCJD brain than has previously been reported. We also discriminated between plasma and serum samples from scrapie-infected and uninfected hamsters, even in early preclinical stages. This combined assay, which we call “enhanced QuIC” (eQuIC), markedly improves prospects for routine detection of low levels of prions in tissues, fluids, or environmental samples. IMPORTANCE Transmissible spongiform encephalopathies (TSEs) are largely untreatable and are difficult to diagnose definitively prior to irreversible clinical decline or death. The transmissibility of TSEs within and between species highlights the need for practical tests for even the smallest amounts of infectivity. A few sufficiently sensitive in vitro methods have been reported, but most have major limitations that would preclude their use in routine diagnostic or screening applications. Our new assay improves the outlook for such critical applications. We focused initially on blood plasma because a practical blood test for prions would be especially valuable for TSE diagnostics and risk reduction. Variant Creutzfeldt-Jakob disease (vCJD) in particular has been transmitted between humans via blood transfusions. Enhanced real-time quaking-induced conversion (eRTQ) provides by far the most sensitive detection of vCJD to date. The 15B3 antibody binds prions of multiple species, suggesting that our assay may be useful for clinical and fundamental studies of a variety of TSEs of humans and animals. Transmissible spongiform encephalopathies (TSEs) are largely untreatable and are difficult to diagnose definitively prior to irreversible clinical decline or death. The transmissibility of TSEs within and between species highlights the need for practical tests for even the smallest amounts of infectivity. A few sufficiently sensitive in vitro methods have been reported, but most have major limitations that would preclude their use in routine diagnostic or screening applications. Our new assay improves the outlook for such critical applications. We focused initially on blood plasma because a practical blood test for prions would be especially valuable for TSE diagnostics and risk reduction. Variant Creutzfeldt-Jakob disease (vCJD) in particular has been transmitted between humans via blood transfusions. Enhanced real-time quaking-induced conversion (eQuIC) provides by far the most sensitive detection of vCJD to date. The 15B3 antibody binds prions of multiple species, suggesting that our assay may be useful for clinical and fundamental studies of a variety of TSEs of humans and animals.

Bioactive alkaloids of frog skin: Combinatorial bioprospecting reveals that pumiliotoxins have an arthropod source

Nearly 500 alkaloids have been detected in skin extracts from frogs of the family Dendrobatidae. All seem to have been sequestered unchanged into skin glands from alkaloid-containing arthropods. Ants, beetles, and millipedes seem to be the source of decahydroquinolines, certain izidines, coccinellines, and spiropyrrolizidine oximes. But the dietary source for a major group of frog-skin alkaloids, namely the pumiliotoxins (PTXs), alloPTXs, and homoPTXs, remained a mystery. In hopes of revealing an arthropod source for the PTX group, small arthropods were collected from eight different sites on a Panamanian island, where the dendrobatid frog (Dendrobates pumilio) was known to contain high levels of two PTXs. The mixed arthropod collections from several sites, each representing up to 20 arthropod taxa, contained PTX 307A and/or alloPTX 323B. In addition, the mixed arthropod collections from several sites contained a 5,8-disubstituted indolizidine (205A or 235B), representing another class of alkaloids previously unknown from an arthropod. An ant alkaloid, decahydroquinoline 195A, was detected in the mixed arthropod collections from several sites. Thus, “combinatorial bioprospecting” demonstrates that further collection and analysis of individual taxa of leaf-litter arthropods should reveal the taxa from which PTXs, alloPTXs, and 5,8-disubstituted indolizidines are derived.

New generation QuIC assays for prion seeding activity

The ability of abnormal TSE-associated forms of PrP to seed the formation of amyloid fibrils from recombinant PrPSen has served as the basis for several relatively rapid and highly sensitive tests for prion diseases. These tests include rPrP-PMCA (rPMCA), standard quaking-induced conversion (S-QuIC), amyloid seeding assay (ASA), real-time QuIC (RT-QuIC) and enhanced QuIC (eQuIC). Here, we summarize recent improvements in the RT-QuIC-based assays that enhance the practicality, sensitivity and quantitative attributes of assays QuIC and promote the detection of prion seeding activity in dilute, inhibitor-laden fluids such as blood plasma.

Human variant Creutzfeldt-Jakob disease and sheep scrapie PrP(res) detection using seeded conversion of recombinant prion protein.

The pathological isoform of the prion protein (PrP(res)) can serve as a marker for prion diseases, but more practical tests are needed for preclinical diagnosis and sensitive detection of many prion infections. Previously we showed that the quaking-induced conversion (QuIC) assay can detect sub-femtogram levels of PrP(res) in scrapie-infected hamster brain tissue and distinguish cerebral spinal fluid (CSF) samples from normal and scrapie-infected hamsters. We now report the adaptation of the QuIC reaction to prion diseases of medical and agricultural interest: human variant Creutzfeldt-Jakob disease (vCJD) and sheep scrapie. PrP(res)-positive and -negative brain homogenates from humans and sheep were discriminated within 1-2 days with a sensitivity of 10-100 fg PrP(res). More importantly, in as little as 22 h we were able to distinguish CSF samples from scrapie-infected and uninfected sheep. These results suggest the presence of prions in CSF from scrapie-infected sheep. This new method enables the relatively rapid and sensitive detection of human CJD and sheep scrapie PrP(res) and may facilitate the development of practical preclinical diagnostic and high-throughput interference tests.

Prion Shedding from Olfactory Neurons into Nasal Secretions

This study investigated the role of prion infection of the olfactory mucosa in the shedding of prion infectivity into nasal secretions. Prion infection with the HY strain of the transmissible mink encephalopathy (TME) agent resulted in a prominent infection of the olfactory bulb and the olfactory sensory epithelium including the olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs), whose axons comprise the two olfactory cranial nerves. A distinct glycoform of the disease-specific isoform of the prion protein, PrPSc, was found in the olfactory mucosa compared to the olfactory bulb, but the total amount of HY TME infectivity in the nasal turbinates was within 100-fold of the titer in the olfactory bulb. PrPSc co-localized with olfactory marker protein in the soma and dendrites of ORNs and VRNs and also with adenylyl cyclase III, which is present in the sensory cilia of ORNs that project into the lumen of the nasal airway. Nasal lavages from HY TME-infected hamsters contained prion titers as high as 103.9 median lethal doses per ml, which would be up to 500-fold more infectious in undiluted nasal fluids. These findings were confirmed using the rapid PrPSc amplification QuIC assay, indicating that nasal swabs have the potential to be used for prion diagnostics. These studies demonstrate that prion infection in the olfactory epithelium is likely due to retrograde spread from the olfactory bulb along the olfactory and vomeronasal axons to the soma, dendrites, and cilia of these peripheral neurons. Since prions can replicate to high levels in neurons, we propose that ORNs can release prion infectivity into nasal fluids. The continual turnover and replacement of mature ORNs throughout the adult lifespan may also contribute to prion shedding from the nasal passage and could play a role in transmission of natural prion diseases in domestic and free-ranging ruminants.

In Vitro Detection of prionemia in TSE-Infected Cervids and Hamsters

Blood-borne transmission of infectious prions during the symptomatic and asymptomatic stages of disease occurs for both human and animal transmissible spongiform encephalopathies (TSEs). The geographical distribution of the cervid TSE, chronic wasting disease (CWD), continues to spread across North America and the prospective number of individuals harboring an asymptomatic infection of human variant Creutzfeldt-Jakob Disease (vCJD) in the United Kingdom has been projected to be ~1 in 3000 residents. Thus, it is important to monitor cervid and human blood products to ensure herd health and human safety. Current methods for detecting blood-associated prions rely primarily upon bioassay in laboratory animals. While bioassay provides high sensitivity and specificity, it requires many months, animals, and it is costly. Here we report modification of the real time quaking-induced conversion (RT-QuIC) assay to detect blood-borne prions in whole blood from prion-infected preclinical white-tailed deer, muntjac deer, and Syrian hamsters, attaining sensitivity of >90% while maintaining 100% specificity. Our results indicate that RT-QuIC methodology as modified can provide consistent and reliable detection of blood-borne prions in preclinical and symptomatic stages of two animal TSEs, offering promise for prionemia detection in other species, including humans.

Prion Seeding Activities of Mouse Scrapie Strains with Divergent PrPSc Protease Sensitivities and Amyloid Plaque Content Using RT-QuIC and eQuIC

Different transmissible spongiform encephalopathy (TSE)-associated forms of prion protein (e.g. PrPSc) can vary markedly in ultrastructure and biochemical characteristics, but each is propagated in the host. PrPSc propagation involves conversion from its normal isoform, PrPC, by a seeded or templated polymerization mechanism. Such a mechanism is also the basis of the RT-QuIC and eQuIC prion assays which use recombinant PrP (rPrPSen) as a substrate. These ultrasensitive detection assays have been developed for TSE prions of several host species and sample tissues, but not for murine models which are central to TSE pathogenesis research. Here we have adapted RT-QuIC and eQuIC to various murine prions and evaluated how seeding activity depends on glycophosphatidylinositol (GPI) anchoring and the abundance of amyloid plaques and protease-resistant PrPSc (PrPRes). Scrapie brain dilutions up to 10−8 and 10−13 were detected by RT-QuIC and eQuIC, respectively. Comparisons of scrapie-affected wild-type mice and transgenic mice expressing GPI anchorless PrP showed that, although similar concentrations of seeding activity accumulated in brain, the heavily amyloid-laden anchorless mouse tissue seeded more rapid reactions. Next we compared seeding activities in the brains of mice with similar infectivity titers, but widely divergent PrPRes levels. For this purpose we compared the 263K and 139A scrapie strains in transgenic mice expressing P101L PrPC. Although the brains of 263K-affected mice had little immunoblot-detectable PrPRes, RT-QuIC indicated that seeding activity was comparable to that associated with a high-PrPRes strain, 139A. Thus, in this comparison, RT-QuIC seeding activity correlated more closely with infectivity than with PrPRes levels. We also found that eQuIC, which incorporates a PrPSc immunoprecipitation step, detected seeding activity in plasma from wild-type and anchorless PrP transgenic mice inoculated with 22L, 79A and/or RML scrapie strains. Overall, we conclude that these new mouse-adapted prion seeding assays detect diverse types of PrPSc.

Quaking-Induced Conversion Assays for the Detection and Diagnosis of Prion Diseases

The seeding activity of prions has been exploited for the development of a number of ultrasensitive assays for transmissible spongiform encephalopathies (TSEs). Among the more practical assays are those that use recombinant PrPSen (rPrPSen) as a substrate for prion-seeded conversion into amyloid fibrils, shaking rather than sonication, and fluorescence detection in multiwell plates. These include the amyloid seeding assay (ASA), real-time QuIC (RT-QuIC), and enhanced QuIC (eQuIC). Recent applications of RT-QuIC to the antemortem diagnosis of sporadic Creutzfeldt–Jakob disease (sCJD) using cerebrospinal fluid (CSF) showed improved specificity (nearly 100%) relative to assays for other CSF markers. Moreover, the RT-QuIC can be quantitative and as sensitive as animal bioassays, allowing measurements of prion seeding activity in CSF and nasal fluids. In hamster CSF, the time course of seeding activity accumulation can vary markedly with route of scrapie inoculation. To enhance sensitivity and to cope with inhibitor-laden sample types such as blood plasma, an immune capture step was integrated with RT-QuIC, giving rise to the eQuIC assay. eQuIC can detect up to 1014-fold dilutions of human CJD brain homogenate in blood plasma and discriminate plasma samples from scrapie-infected and uninfected rodents. Although further work is required to fully validate various applications of RT-QuIC, eQuIC, and related tests, these assays improve prospects for practical prion detection in humans, animals, biomaterials, and the environment.